1. Field of the Invention
The present invention relates to a digital television broadcast signal receiver for receiving a digital television broadcast signal, e.g., of ATSC format.
2. Description of the Related Art
In the North America, ATSC digital television broadcasting services (referred to as TV broadcast hereinafter) are widely provided from their stations, commonly located at large cities or their suburbs in the plains and their TV broadcast signals are hence received from all directions by each user who has to inevitably direct its antenna towards the stations transmitting TV broadcast signals of desired TV programs. For compensation, there have been developed and nearly marketed a number of multi-directional antennas including smart antenna. One of the most known antennas for receiving ground television broadcast signals is Yagi antennas. In particular, Yagi antennas are highly directional and can thus receive as low signals as possible. However, as Yagi antennas are highly directional, they may receive only the signals from a small group of stations. Also, Yagi antennas may vary largely in the intensity of a received signal as they are shifted from one direction to another.
There have been introduced some techniques for controlling the receiving direction of an antenna; a method of determining the optimum receiving condition of an antenna with the use of a sensor which measures the angle of turning of the antenna (Japanese Patent Laid-open Publication 05-267919), a method of controllably turning an antenna, e.g., vehicle-loaded satellite broadcast signal receiving antenna, through providing a driving signal and a directional signal in response to the level of an received signal (Japanese Patent Laid-open Publication 06-225230), and a method of modifying the turning speed of an antenna, e.g., vehicle-loaded satellite broadcast signal receiving antenna, in response to a change in the intensity of a received signal (Japanese Patent Laid-open Publication 2003-344517).
As described above, a multi-directional antenna (such as smart antenna) is preferable for receiving a plurality of aerial signals of the ATSC digital television broadcast service which are radiated from different directions. The EIA-909 standard determines that the receiving direction of smart antennas shall respond to each of directions determined through dividing the full circle by 16. A digital television broadcast signal receiver is thus arranged for receiving TV broadcast signals from all the sixteen discrete directions with its multi-directional antenna positioning control as conforming to the EIA-909 standard.
Some of digital television broadcast signal receivers (referred to as digital TV signal receivers hereinafter) are equipped with an auto scan function for selecting the receiving direction of an antenna in which the TV signal at a user-desired channel is received at optimum. As the antenna or transmitter tower of a broadcast station is not movable, the direction in which the received signal is at optimum can be predetermined through scanning the sixteen directions at each channel. However, In the U.S.A., a plurality of new broadcast stations are established with comparatively easiness while some of the existing broadcast stations are eliminated. Hence, there may frequently happen that the existing channel is suddenly lost while new channels are introduced. Also, a high skyscraper is built at the neighbor area and will interrupt the reception of TV broadcast signals. This requires the user to reset the direction of the antenna in which the TV signal at each channel is received at optimum. Even when the digital TV broadcast signal receiver equipped with an auto scan function is connected with a multi-directional antenna arranged for minimizing the time required for resetting the receiving direction of the antenna, the interval between outputs of the control signal for modifying the direction of the antenna is not short, thus increasing the time required for determining the direction in which the TV signal at each channel is received at optimum and irritating the user.
When the antenna installed in a room is interrupted by someone walking across its direction, its received signal intensity may temporarily drop down. Also, any flying object, such as a helicopter, flying low may interrupt the signal reception of the antenna in a moment and lowers the intensity of the received signal. The smart antenna is arranged for constantly operating a turning movement to search the direction in which the signal is received at optimum and can quickly respond to any drop down in the intensity of the received signal to shift back the intensity to its original level. However, when the intensity of the received signal has been returned back to its original level instantaneously in response to the drop down, the antenna operates its full turn and temporarily faces another direction as dislocated from the direction where the received signal is at optimum. As the intensity of the received signal is downwardly overshot (See FIG. 7), the reproduction of a TV image from the received signal will be interrupted. It is hence needed for compensating the overshooting to operate the antenna again and thus waste another time and energy.